Enhancement: add constants for IPv4 and IPv6 subnet masks

Rationale: This SO question demonstrates there is at least some need to manipulate netmasks into CIDR integers. The accepted answer contains a bug which can be easily eliminated by hard coding the 2 sets of netmasks into the IPAddr class - see my alternative answer.

Proposed solution: Define IPAddr::IPV4_SUBNET_MASKS and IPAddr::IPV6_SUBNET_MASKS as follows:

class IPAddr
  IPV4_SUBNET_MASKS = (0..32).map { |n| IPAddr.new("0.0.0.0/#{n}").netmask }
  IPV6_SUBNET_MASKS = (0..128).map { |n| IPAddr.new("0::/#{n}").netmask }
end

# Now a user can do the following to get a subnet mask from a CIDR integer:
IPAddr::IPV4_SUBNET_MASKS[24]
# => "255.255.255.0"

IPAddr::IPV6_SUBNET_MASKS[64]
# => "ffff:ffff:ffff:ffff:0000:0000:0000:0000"

Optional: Add a predicate to check if an instance of IPAddr is a netmask:

class IPAddr
  def netmask?
    if ipv4?
      IPV4_SUBNET_MASKS.include? to_s
    else
      IPV6_SUBNET_MASKS.include? to_s
    end
  end
end

ipaddr1 = IPAddr.new '255.254.255.0'
ipaddr1.netmask? # => false

ipaddr2 = IPAddr.new '255.255.255.0/24'
ipaddr2.netmask? # => true

ipaddr3 = IPAddr.new '255.255.255.0'
ipaddr3.netmask? # => true

# And so on for IPv6 addresses

IPAddr クラスから始め、既存の IPv4、IPv6、およびネットマスクの処理を調べます。提案されているサブネットマスク定数のテストカバレッジを追加し、オプションの述語を含める場合は、その IPv4 と IPv6 のケースも追加します。完了の条件は、CIDR インデックスが文書化されたマスクを返し、非連続マスクが拒否されることです。